Optical waveguide fibers are normally provided with a plastic protective coating applied by the fiber manufacturer directly over the cladding in order to protect the fiber.
In tight buffered cables, a protective tube is applied by the cable manufacturer directly to the outer surface of the protective fiber coating so that the fiber and the tube are in contact substantially along their length. Such an optical fiber cable is known from U.S. Pat. No. 5,627,932.
In loose tube or composite buffer constructions, the coated optical fiber is separated from the protective tube. Such a cable is known from U.S. Pat. No. 6,091,871. This patent discloses a reinforced optical fiber cable that includes a protective tube for protecting optical fibers, a reinforcing layer, and reinforcing rods around the protective tube, together with an outer sheath. Such a cable has relatively stiff rods that are embedded in the sheath, parallel to the axis of the cable. These rods may be made of reinforced plastic material or steel. In addition, the inside space in which the optical waveguides are received contains a filler material to protect the optical waveguides against penetration of moisture.
The SEDI FIBRES OPTIQUES PRODUCT CATALOG, February (2002 2002-02, France) discloses a loose tube optical waveguide fiber assembly, but it is made of material that is not flame retardant.
The U.S. website for CORNING CABLE SYSTEMS shows its “ALTOS LSZH Gel-Free Interlocking Armored Cables” as originally disclosed in March 2006. (On Feb. 21, 2007, this document was available on the Internet at URL http://www.corningcablesystems.com/web/library/lltindex.nsf/$all/LAN-491-EN/$file/LAN-491-EN.pdf>). The Corning website discloses a loose tube optical waveguide fiber assembly made of a material that is not flame retardant, although the tubes are arranged inside an outer flame-retardant sheath. The space between the optical waveguide fibers and the tube is filled with swellable materials.
Publication No. GB 1,529,001 relates to an optical conductor for use in an optical cable having a glass fiber and a protective sheath surrounding the glass fiber. The protective sheath is composed of two layers of different synthetic resin materials (i.e., an inner layer consisting of polystyrene or a fluorinated polymer, and an outer layer consisting of a polyamide, a polyterephthalate, a polypropylene, or polyethylene).
U.S. Pat. No. 6,167,178 relates to a fiber optic cable including at least one optical fiber having a buffer layer formed of a flame-retardant polyolefin material that is tightly formed about the optical fiber, thereby defining a tight buffer layer, a layer of strength members, and a cable jacket surrounding optical fiber in contact with at least some strength members.
Such tight buffered cables, however, are expensive and accessing the fibers is time-consuming. Central-filled loose tubes are expensive, too, and require fiber cleaning. In addition, these gel-filled central loose tubes have high minimum cable bend radii due to kinking of the loose tube.